Prior to patch 1.2 you can bribe anything you want, treasury pending

After 1.2 patch
If your Roman you can't bribe Roman generals until the civil war starts. You can bribe any other of your allies general.

Not to sure if your non Roman and allied to a Roman if you can bribe thier generals. So basically if thet're not Roman they are free game for sure.

Also bribing amounts to transgression wich provokes a higher chance of them attacking you.